Field of the Invention
This invention is directed to a method for mixing lithium hexafluorophosphate (LiPF.sub.6) salt into an electrolyte solution suitable for use in preparing a solid electrolyte while inhibiting decomposition of this salt.
State of the Art
Batteries typically comprise a group of electrically connected electrolytic cells which cells individually comprise a cathode, an anode and interposed therebetween, an electrolyte. The electrolyte can be formulated as a conducting liquid solution or as a conducting solid composition each of which contains an electrolytic salt. When the electrolyte is a conducting liquid solution, the resulting battery containing such electrolytic cells is often referred to as a "liquid" battery; and when the electrolyte is a conducting solid composition, the resulting battery containing such electrolytic cells is often termed a "solid" battery.
The electrolytic cells of solid batteries typically employ an anode comprising lithium and, accordingly, the electrolytic salt employed in the electrolyte of such solid batteries is a typically a lithium salt. Suitable lithium salts for use in such solid batteries containing a lithium anode are well known in the art and include, by way of example, LiI, LiBr, LiSCN, LiClO.sub.4, LiBF.sub.4, LiPF.sub.6, LiAsF.sub.6, CF.sub.3 CO.sub.2 Li, CF.sub.3 SO.sub.3 Li, and the like, however, LiAsF.sub.6 is conventionally employed in such solid batteries.
The solid electrolyte is conventionally prepared by combining a prepolymer, the LiAsF.sub.6 salt and optionally, a film forming agent such as polyethylene oxide into an electrolyte solvent wherein the resulting solution is mixed until all of the components are dissolved. The resulting electrolyte solution is then coated onto the surface of a substrate (e.g., the cathode or anode) and then cured so as to polymerize the prepolymer which converts the composition from a liquid to a solid thereby forming a solid electrolyte.
While the solid electrolyte prepared by such conventional techniques provides for electrolytic cells useful in generating a solid battery, the use of LiAsF.sub.6 as the electrolytic salt in solid electrolytes is particularly disadvantageous because this salt is toxic and this toxicity raises environmental concerns regarding the safe disposal of spent solid batteries containing this salt.
Partly to avoid this problem, this invention employs LiPF.sub.6 as the electrolytic salt because this salt is less toxic than LiAsF.sub.6 and provides for improved capacity (improved cycle life) and approximately the same conductivity as compared to LiAsF.sub.6. See, for example, U.S. patent application Ser. No. 08/049,212 filed concurrently herewith as Attorney Docket No. 1256 and entitled "Compositions and Methods for Improving the Cumulative Capacity of Solid, Secondary, Electrolytic Cells", which application is incorporated herein by reference in its entirety.
However, in preparing solid electrolytes containing LiPF.sub.6, it was found that the addition of the LiPF.sub.6 into the electrolyte solvent was accompanied by a temperature increase in the solvent due to the heat of solvation of this salt which typically increased the temperature of the mixture. Moreover, it was further found that this temperature increase and/or the application of heat during mixing was sufficient, in some cases, to decompose a significant portion the LiPF.sub.6 salt which was evidenced by a change in the color of the clear solution through a straw yellow to a dark brown. Without being limited by any theory, it is believed that this decomposition reaction proceeds as follows: ##STR1##
It is further believed that the so generated PF.sub.5 can react with any ethers contained in the solution (e.g., the polyethylene oxide film forming agent and the electrolyte solvent if the solvent contains ether bonds) by breaking the ether bonds and causing further degradation of the electrolyte solution.
On the other hand, some heat is required in the process of combining the components used to prepare the solid electrolyte because the polyethylene oxide film forming agent typically softens (melts) at a temperature of about 55.degree. C. to 65.degree. C. and softening of the polyethylene oxide is required to initiate its dissolution into the electrolyte solvent.
In the past, liquid electrolyte solutions have avoided the thermal instability problems of LiPF.sub.6 by first complexing the LiPF.sub.6 with dimethoxyethane (DME), isolating the resulting adduct, and then using this adduct to form the liquid electrolyte solution. See, for example, Bowden, U.S. Pat. No. 4,880,714. Apparently, by first complexing with DME, the heat of solvation can be controlled. However, liquid electrolyte solutions do not employ polyethylene oxide film forming agents and this approach does not solve the problem of solubilizing the polyethylene oxide film forming agent without causing significant decomposition of the lithium hexafluorophosphate salt.
In view of the above, the inventors were confronted with a problem that formulating the solution used to prepare the solid electrolyte composition required heat to dissolve the polyethylene oxide film forming agent used in this solution and that this heat would, in some cases, also decompose the lithium hexafluorophosphate salt.